Telephone service within buildings is provided by riser cables which generally extend from vaults in basements to the floors above. Because of the environment in which these riser cables are used, they must meet specified requirements which relate to fire-retardancy. One measure of fire-retardancy is a parameter which is known as the limiting oxygen index. That parameter is a function of the materials which comprise the cable, their surface areas and their structural arrangement. Another parameter, fuel content, is intended to mean that quantity of fuel which is released by the materials comprising the insulation and the jacketing after a fire starts.
Typically, a riser cable includes a core having a plurality of twisted pairs of conductors which are individually enclosed wih a composite unexpanded insulation comprising a polyvinyl chloride skin that is extruded over a solid polyethylene inner layer. The twisted pairs of conductors are enclosed in a sheath which is identified by the acronym ALVYN. The ALVYN sheath comprises a polyvinyl chloride jacket that is bonded to a corrugated aluminum shield.
The just-described insulation structure combines the acceptable fire-retardant characteristics of polyvinyl chloride and the superior dielectric constant of polyethylene. As a result, riser cables having required transmission characteristics such as a mutual capacitance of 52 nf/kilometer can be achieved within a reasonable cable diameter range. However, this insulation which contains approximately 50% polyethylene by weight is relatively high in fuel content.
Another consideration is the pair count density, which is the number of insulated conductors in a given cable cross-section. With the trend toward larger and larger buildings and the increased use of the telephone for various kinds of communication, the pair count density within a building riser system generally must be greater than that achieved in the past.
Also of importance to building cables is the capability of color coding the conductor insulation. Typically, a predetermined number of conductor pairs are grouped together in what is referred to as a unit. The unit is characterized by unique color combinations among the pairs as well as a binder having a particular color. This allows an installer to be able to identify a particular conductor pair and to distinguish between tip and ring. As a result of the relative ease of identification, splicing and termination costs are greatly reduced.
A number of jacket and insulation systems are well-known in the art, but none that are known meet all the above-mentioned requirements. For example, it is known that polyvinyl chloride is a fire-retardant material, and that polyethylene has an excellent dielectric constant which is helpful to the transmission qualities of the cable. Expanded polyethylene has a lower dielectric constant which leads to the optimization of the cable size and which is somewhat thermally insulating, that is, it limits fire spread. Pulp insulation lends itself well to high pair density cable systems, but it does not lend itself to the color coding scheme which is desired for inside wiring and splicing.
As for the prior art, O. G. Garner U.S. Pat. No. 3,378,628 discloses a dual insulated telephone wire which is suitable for use in outside plant cables as well as inside buildings. The insulation comprises an inner layer of solid or expanded polyethylene while the skin is disclosed to be a fire-retardant material such as polyvinyl chloride. While the patent identifies alternate insulation systems for use in riser cables, it does not address the multi-faceted problem that must be overcome today. For example, a less than complete consideration of all the parameters which are involved could result in a cable design having an un acceptably high fuel content which exacerbates rather than solves the problem.
What is needed is an insulation and jacketing system which minimizes the opportunity for the beginning of a fire along a riser cable, and should such a flame be initiated, one which minimizes the propagation of the flame and the total heat which is released by the cable system. Also, the conductors must have a relatively small diameter-over-dielectric in order to reduce the outside diameter of the cable, but must also exhibit acceptable transmission characteristics. Lastly, the outer insulation must lend itself to a color coding scheme in order to facilitate inside wiring and splicing. Seemingly, these needs have not been met by the prior art including the Garner patent.